Loft conversion costs 2021 and how to successfully plan your project

This is everything you need to know about loft conversion costs this year. We talk you through planning and budgeting for your home improvement project in 2021

Loft conversion: If you have space up top, use it to create extra rooms, or a stunning master suite
(Image credit: Econoloft)

When you're fighting for more room, and when loft conversion costs can start at as little as £15,000, this home improvement project is a no-brainer. Straightforward in most cases and a sound alternative to moving altogether, let our guide help you plan and budget for your house renovation project so that it is a complete success.

Since most loft conversions are generally allowed under permitted development rights, there’s no need to go through the lengthy process of obtaining planning permission. And, another bonus is that you'll get the extra room you need, without having to sacrifice garden space, as is the case a conventional rear or side extension. 

In this feature we go through all the detail behind the cost of loft conversions, as well as how to plan your loft conversion and thoughts on developing your space for the best result.

How much will a loft conversion cost in 2021?

Loft conversion costs will vary depending on size, whether you'll need to alter its structure for staircase access, the type of conversion you're going for and where you live in the country, but as an average, loft conversion costs come anywhere between £30,000 and £50,000

To break loft conversion costs down further, identify the type you are going for:

A basic, room in the roof conversion: for the simplest of loft conversions, usually involving floor reinforcement, skylight(s), insulation, a staircase, electrics, lighting and heating plus fire safety measures, loft conversion prices can start from around £15,000 (or around £1,150-£1,350 per square metre). 

A dormer loft extension: including all the provisions above, plus a dormer window. This might be your next best option if the basic rooflight conversion doesn't offer enough head height or floor space. Expect to pay from £20,000 for a basic conversion; for the inclusion of an en suite and a finish suited to a master suite, you're more likely to pay £35,000 upwards (or around £1,250-£1,450 per square metre).

A mansard extension: expect to pay £35,000 to £45,000 upwards (from £1,500-£2,500 per square metre).

A hip-to-gable extension (see page two for more on this roof type): expect to pay £25,000 to £30,000 upwards (costs range from £1,500-£2,500 per square metre).

Ready-made loft conversions, made off-site and craned into position are quick and will reduce labour costs, which can be beneficial if you live in an area where labour costs are higher. Expect to pay around £55,000.

A conversion that requires change of roof structure: this is obviously the most costly of options, since removing and rebuilding the roof will require an experienced designer and increase labour and material costs. This type of conversion is likely to cost upwards of £40,000 (or from £1,350-£1,550 per square metre).

Do loft conversions add value?

Loft conversions provide one of the best returns on investment you can get when it comes to extending, with experts suggesting they add up to 20% to your home (more on this below). To make it worth it, the cost of loft conversion shouldn't be greater than the added value to your property, so do the maths and be sure to check out the ceiling price for properties in your area to avoid disappointment before you start.

How to get a good return on investment

Adding a room to your house should increase its value by between £50,000 and £60,000. As most loft conversion prices only cost between £30,000 and £40,000 to complete, this is a considerable return on investment. Michael Holmes advises on budgeting:

  • £1,150–£1,350 per square metre for a basic rooflight conversion
  • £1,250–£1,450 per square metre for a dormer conversion
  • £1,350–£1,550 per square metre for complex options

You can find a more accurate estimation for your loft conversion with the help of our extension cost calculator.

bed in a converted chapel with storage built around it

(Image credit: Chris Humphreys)

Additional loft conversion costs – planning and professional fees

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Loft extensions can usually be completed under permitted development rights, but it is worth obtaining a Certificate of Lawfulness (£86) from your local council for the work. 

Should planning be required, expect planning permission fees (£172); these may be included in your agreed contract with the loft company, but do check. 

If you live in an attached house, you will need a party wall agreement with your neighbour(s). These typically costs from £700 to £1,000 per neighbour.

Building control fees (around £500, plus VAT) are also payable by the homeowner to the local authority or a government-approved, independent inspection company, to check that the work is as contracted and to issue building regulation certificates to prove that it has been carried out in accordance.

Find out more about planning permission, permitted development, party wall agreements and building regulations on page two.

You’ll find small companies will usually charge 10-15 per cent less than large companies.

Home office storage with blue cupboards and open shelves

(Image credit: Neville Johnson)

Additional loft conversion costs – design fees

It's likely that design fees are included within your loft company's quote, but if you've appointed an architect to design your loft conversion, expect design fees from three to seven per cent of the construction cost, with a minimum of around £2,700 to £4,000 for the planning drawings, with construction drawings costing a similar. If you need a structural engineer to size roof joists, budget for £500 to £1,000.

If you have asked your architect to put the contract out to tender, appoint the builder and administer the contract, budget for a further three to seven per cent of the build cost. Or, you can agree a day or hourly rate for ad hoc site attendance.

The interior fittings of your loft extension will usually include the basics: sanitaryware; electric points and basic light fittings; door and window furniture; skirting and woodwork; possibly flooring. The price may also include basic decoration – often an all-over spray-paint with a white finish for walls and woodwork. For anything extra, you will need to ask for quotes on top of the original price.

The cost of loft conversion vs what you should spend

Given that a loft extension costs upwards of £30,000 (but often much more, particularly in large cities and in larger properties), there is a limit to how much you should spend on your loft conversion. 

As aforementioned, if you plan on eventually selling the house, you will need to consider the ceiling price of your street – an estate agent can advise on this. Essentially, you want the value of your house to increase by at least the cost of your entire loft conversion, but by spending too much, you may over-value your own house, making it difficult to sell for an appropriate profit.

Comparing the quoted cost of your loft conversion, plus the value of your home, with the cost of moving to a larger house in the same area is a worthwhile practice in assessing the benefits of converting the space.

For example:

Your house is valued at £270,000.

The loft conversion costs £50,000, making the projected cost of the home £320,000.

However, the ceiling price of your street is £300,000, making it impossible to recoup the £20,000 excess spent on a loft conversion.

If there is a house in your area with the space you require for less than £320,000, it is worth considering a move, rather than investing in the loft conversion.

Planning a loft conversion

Planning a loft conversion involves everything from getting the right paperwork signed off, deciding on roof type, how the staircase will reach the new room and choosing the insulation. Of course, some decisions will be made for you by your loft company – but it's always worth getting ahead of the curve so that when they ask you for that piece of paper or what type of material you want for your window frames, you're ready. Follow our tips below for planning your loft extension.

Planning a loft conversion: is your loft suitable for conversion?

Most properties will be suitable for a loft conversion so long as they have a loft that measures 2.3m at the highest point. As well as head height, other features that will help you decide whether your loft space is suitable for conversion are the pitch of the roof, the type of structure, and any obstacles, such as water tanks or chimney stacks.

Is there enough roof height?

If the initial roof space inspection reveals a maximum head height of less than 2.3 metres, there are two solutions available, both of which will require professional input: You could remove all or part of the roof and rebuild it to the required height and structure; however, this is costly and requires getting planning permission. You’ll also need to protect your house from the weather during the works using a covered scaffold structure.

Alternatively, you could create height by lowering the ceiling of the room below, providing you maintain a height of at least 2.4m. Removing the existing ceilings is a messy job and a plate will need to be bolted to the wall for the new floor joists to hang from. There will also need to be a tie between the new ceiling and roof to prevent the roof spreading.

Is the pitch suitable?

The higher the pitch angle, the higher the central head height is likely to be. If dormers are used or the roof is redesigned, the floor area can be widened.

Loft bedroom with en suite in a 15th century converted mill

(Image credit: Solebury & Worthy)

How long does a loft conversion take?

A typical loft conversion project will take four to six weeks to complete and most of the work can be done without disturbing the existing house. A more complex project that involves removing the existing roof will take eight to 10 weeks.

Which type of loft conversion is best for your home?

There are four fundamental types of loft conversion and they vary in complexity and in cost as noted above.

Turning an attic into a room with rooflights

The existing loft space can be converted by simply adding rooflights, such as Velux windows, plus upgrading the structure and adding stairs, electrics, plumbing, insulation. This is usually the simplest, quickest and cheapest type of conversion, as structural alterations are kept to a minimum. 

Rooflights are also the most straightforward way of adding natural light and ventilation to your loft conversion. The surrounding area is reinforced before the rafters are cut to make way for the rooflights. The rooflight frame is fitted within the opening, and flashings are added before making good the surrounding tiling. This type of window is the most cost-effective, and most likely to be allowed without planning permission, under permitted development rights. Conservation rooflights, which are slightly more flush to the roofline and made of metal, can also be installed.

The cross section of a roof light loft conversion

The cross section of a roof light loft conversion

Dormer loft conversions

Dormers not only give natural light, but can add space to a loft conversion, too. They are particularly effective where the pitch angle is high, as they can help increase the useable floor space. Dormers are normally installed by opening up the roof and cutting the required timbers to size on site. However, some loft conversion companies will make the dormers off site in their workshop and lift into place, which allows quick installation, and weatherproofing.

There are various types of dormer, from the standard ‘box’ which projects out with a flat roof, to the ‘hip-to-gable’ (see more below), which is used on end-terrace or semi-detached houses to replace a previously sloping roof (a hip) with a wall that is flush to the exterior wall (forming a gable). The mansard type, most commonly seen on London terrace houses, also maximises available roof space because it projects the maximum available head height, giving a greater usable floor area.

The part of the roof being extended will need to be stripped and the structure rebuilt. Consequently, this option is more time-consuming and expensive. 

The cross section of a dormer loft conversion

The cross section of a dormer loft conversion

Creating a new mansard roof

This is where the roof structure is altered at the back of the house (and sometimes at the front, too) to create a far larger area with full headroom. A mansard conversion typically spans from gable wall to gable wall and is like another full storey with almost vertical tiled walls and a flat roof. This results in an addition that may appear far more a part of the property and less like an add-on than a large box dormer. 

The cross section of a mansard loft conversion

The cross section of a mansard loft conversion

Hip to gable loft conversions

This usually applies to a semi-detached house or bungalow where the roof is currently hipped (sloped) to the side, as well as to the back and front. This roof is stripped and the hipped section removed. The end wall is then built up to form a new vertical gable and a standard pitched roof. The work creates a far greater area with full headroom. 

Gable-end windows

Most loft conversions will have at least one vertical external wall and standard window openings can be formed in these walls to bring in extra light. New windows in side elevations do not usually require planning permission if they are obscured, or are more than 1.7m above floor level.

The cross section of a hip-to-gable loft conversion

The cross section of a hip-to-gable loft conversion

Does a loft conversion need planning permission?

A loft will not usually need planning permission, but always check with your local planning department. As a general rule, loft conversions are classed as permitted development and generally do not require planning permission, providing they meet the following conditions:

  1. Any new roofing must not exceed an additional 40 cubic metres of space on terraced houses.
  2. Any new roofing must not exceed an additional 50 cubic metres of space on detached and semi-detached houses.
  3. No extension must be made beyond the plane of the existing roof slope.
  4. No extension can be higher than the highest part of the roof.
  5. New roofing materials need to be like-for-like or close to original fittings.
  6. There must be no raised platforms or balconies.
  7. Side-facing windows must be set with obscured glazing and an opening 1.7-metres above the floor.
  8. For listed buildings or those in Conservation Areas, visit

Do loft conversions need building regulations approval?

Always remember that planning permission does not equal building regulations approval – the two have to be cleared separately. Every new conversion, including those done under permitted development, must comply with fire and building regulations. This covers the safety and quality of the building work, including:

  • checking the proposed structure is calculated properly;
  • the safety of the stairs;
  • insulation levels;
  • effective drainage;
  • electrical safety.

Ask your local authority’s building control department, or a private sector approved inspector, to help early in the planning stages. A common pitfall is the need for a ‘direct means of escape’, so a separated stairway and hallway with fire-rated doors to all rooms, will be required (see below).

Loft conversion planning: fire safety when you convert a loft

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In an unconverted loft, the plasterboard ceiling in the upstairs rooms will delay the spread of fire to the roof space. However, when an opening is introduced for the staircase, safeguards must be in place to reduce the risk.

Building regulations require loft conversions that are 4.5m above external ground level to have 30-minute fire protection in the floor and walls, including the protected route to an external door. This often means upgrading doors to all habitable rooms on to the stairwell to fire doors. In a bungalow, a fire escape window with a clear opening of at least 45cm x 45cm in each habitable room is acceptable. In an open-plan house, sprinklers may be a suitable alternative.

At least one mains-operated smoke alarm with battery back-up must be installed in the circulation space of each storey.

What if your stairs rise from the living room, not a hallway. Is this permitted?

No, in this case, the stairway must be separated from the rooms by walls and fire doors, leading all the way to an external door, and not open-plan to rooms.

You could create a hallway by partitioning off the stairs from the room, or if the stairs are alongside a spinal wall separating two ground-floor rooms, you could form a lobby at the bottom of the stairs with fire doors leading to each room. So long as you have separate escape routes from both of these rooms, it would be acceptable.

If you really want an open-plan layout to include the stairs, sprinklers are usually the only option.

When converting a loft in a single-storey home, it is permissible to have the stairs within a room as you don’t need a protected stairway. In this situation, the requirement for an alternative emergency fire escape could be met by a first-floor window.

loft conversion with glazed dormer

As part of a whole-house renovation project, Erica and Andy Job decided to open up their loft to create their dream bathroom

Planning a loft conversion: structural considerations 

There are two main types of roof construction – traditional framed and truss section. The traditional framed type is typically found in pre-1960s houses where the rafters, ceiling joists, and supporting timbers are cut to size and assembled on site. This type of structure is usually the most suitable for conversion as it can be easily, and relatively inexpensively, opened up by strengthening the rafters and adding supports.

Post-1960s, the most popular form of roof construction is factory-made truss sections, which mean the entire roof can be erected and felted in a day. Thinner – and therefore cheaper – trusses are used that usually have no loadbearing structures beneath them. Opening up lofts with this kind of structure requires added structural input, most commonly from the addition of steel beams. This requires skill, knowledge and equipment, and is therefore costly.

Planning a loft conversion: will your loft need new joists?

Your existing ceiling joists are unlikely to be able to support the loft conversion floor; so extra joists will need to be added to comply with building regulations. A structural engineer will look at the separation distance needed between joists to support the anticipated load weight, and then specify the size and grade needed.

The new joists will run alongside the existing joists and span between load-bearing walls. They will normally be raised slightly to prevent them from touching the ceiling plaster below.

Above window and door openings, thicker timbers will be used to bridge the gap, so that pressure is not put on the existing lintel. Rolled Steel Joists (RSJs) may also be needed to distribute the load.

(Image credit: Chris Snook)

The amount of useable space for a rooflight conversion will depend on the height and pitch of the roof. By the time the structure has been converted and insulated, only the area measuring 2.3m or more between the top of the floor joists and the underside of the rafters will have enough clear headroom for standing.

If your loft does not yield sufficient space for a simple rooflight conversion, you will need to consider one of the other design options shown on these pages to create the useable new room you need.

Insulation for loft conversions

There are two main ways of insulating the roof structure, and your Building Control inspector will specify which type you require.

Cold roof

The first method, called ‘cold roof’ insulation, can be carried out by a DIYer. It involves filling the space between the rafters with 7cm-thick slab foam insulation, ensuring that there is 5cm space between the roof felt and the insulation to allow for ventilation. A 3cm-depth of slab insulation is then attached to the inside of the rafters, giving a total of 10cm of insulation. The roof section of the loft conversion will require 30cm of mineral wool insulation, or 15cm of slab foam insulation, such as Celotex.

Warm roof

The other main method is ‘warm roof’ insulation. This involves fitting 10cm of slab foam insulation over the top of the rafters and adding a capping, followed by the tile battens and tiles. This is only a practical solution when the roof covering has been stripped off, such as where a dormer is being created. The dormer walls can be insulated with 10cm-thick slab foam insulation between the studwork. Plasterboard is attached to one side of any internal partition walls, a 10cm-thick quilt of insulation added, and then plasterboard added to the other side. Insulation is also placed between floor joists, and this is typically 10cm thick.

If you have space up top, use it to create extra rooms, or a stunning master suite

(Image credit: Clear Architects)

How to choose a staircase for a loft conversion

The best place for your staircase to land is in line with the roof ridge, which will make best use of the height available. The minimum height requirement above a staircase pitch line is two meters. In reality, the actual location of your staircase will depend on the layout of the floor below, and where the necessary height can be achieved using a dormer, rooflight or, if appropriate, converting a hip roof end to a gable.

Number of steps

Building regulations specify that the maximum number of steps you can have in a straight line is 16 – the average loft conversion normally only requires 13 steps.

Size of steps

The maximum step rise is 22cm and the minimum depth is 22mm. Any winders, steps that go around a corner, must have a minimum of 5cm depth at the narrowest point. The width of steps is currently unregulated.

Rules for balustrades

The minimum height of any balustrade is 90cm above the pitch line. Any spindles must have a separation distance that a 10cm sphere cannot pass through.

Heating and ventilating a loft conversion 

Before work begins, check your boiler can cope with the extra demand for heating and hot water. In particular make sure the water pressure is sufficient – you might need to raise the height of the header tank, or switch to a pressurised plumbing system. 

Ensure good natural airflow by placing windows that open at opposite ends of the new room, as loft rooms can get warm.

Decide on the exterior wall cladding

exterior wall cladding

(Image credit: French + Tye)

You’ll want the loft to look great from the outside, too. A dormer can be clad in tiles or slate to match the original roof, finished in timber or with fibre cement weather boarding, which has the same look but is low maintenance and comes in a variety of shades. Metal cladding options include  galvanised steel, aluminium, long-lasting zinc and copper. Allow them to weather naturally, or choose a finish that will keep them gleaming. This loft on a north-London Victorian house designed by Office S&M has distinctive rounded cedar shingles, which will weather and change colour over time, lending it a sense of fun. Lofts come under ‘permitted development’ but materials are required to be similar in appearance to the main house so run your cladding choice by your local planning department in case there are restrictions.

Do you need an architect for a loft conversion?

A loft conversion project is a complex task and it's best to take on professionals rather than tackle it on your own. They will be able to draw up detailed plans that include all the information your local authority Planning Department would need to decipher whether or not you need planning permission.

Choosing a loft conversion company

When choosing a specialist loft company, you will benefit from an integrated project team who all work together to ensure the journey from initial concept to final build is as simple and stress-free as possible. 

The loft company will provide professional architectural designers, qualified Royal Institution of Chartered Surveyors (RICS) structural engineers, surveyors, loft estimators, project managers and specialist tradespeople and loft teams. This structure means that the company is able to see the project as a whole from the outset, giving it the ability to foresee potential problems.

Do not be persuaded to simply opt for the cheapest company. Find a company or person that makes you feel comfortable, and always ask to speak with a previous customer and whether you can pop over to have a look at the work done. Admire the basics, such as how a hinge has been chipped into a new door, or the quality of the new roof and appearance of the new dormer. 

Find out if their chosen company went the extra mile by asking questions. Were they punctual, clean and tidy throughout the build? Did they start and finish the loft conversion within an agreed build schedule? Did they ask for any extra money along the way or was it a fixed price? You will find a lot of loft companies happy to commit to a fully fixed price and to sign documents to reflect this. As they are the professionals, it is their duty to capture all the costs, not yours.

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Lucy Searle
Lucy Searle

Lucy is Editor-in-Chief of, having worked on numerous interiors and property titles. She was founding Editor of Channel 4’s 4Homes magazine, was Associate Editor at Ideal Home. She has also written for Huffington Post, AOL, UKTV, MSN, House Beautiful, Good Homes, and many women’s titles. Find her writing about everything from buying and selling property, self build, DIY, design and consumer issues to gardening.